Submersible pumping units in use heretofore have included a hydraulic motor driving an axial flow pump and both mounted in a casing that is lowered into water. The hydraulic motor is connected through hoses or pipes to an above-ground pump, which pumps oil through the hydraulic motor to drive it, with the return flow of oil going back to an above-ground tank which supplies the oil to the above-ground pump as in U.S. Pat. No. 4,138,202.
The hydraulic motor and the pump in the submersible pumping unit are stopped by turning off the above-ground pump. When this happens, the water in the submersible pump unit tends to flow back down by gravity and rotate the impeller of the submerged pump in reverse. This pump, in turn, tends to drive the hydraulic motor in reverse, causing the latter to pump oil in reverse through the above-ground pump. A gate valve on the pumping unit prevents reverse flow of water through the pump. However, the gate valve may bang shut at shut-down damaging the submersible unit.
An aspect of this invention is concerned with relieving hydraulic surge at start-up and shut-down, and with preventing abrupt closing of the gate valve of the pumping unit at shut-down.